1. Field of the Invention
The present invention relates to a thermal head adjusting method capable of executing thermal recording in constant density without being affected by difference of individual thermal heads in thermal recording apparatuses using a thermal head.
2. Description of the Related Art
Thermal recording materials comprising a thermal recording layer on a substrate such as a film, which are hereunder referred to as thermal materials, are commonly used to record the images produced in diagnosis by ultrasonic scanning.
This recording method, commonly referred to as thermal image recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence, the use of the thermal image recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal image recording involves the use of a thermal head having a glaze in which heat generating resistors constituting heat generating elements and used for heating the thermal recording layer of a thermal material to record an image are arranged in one direction and, with the glaze (heat generating elements) urged at small pressure against the thermal material (thermal recording layer), the two members are relatively moved in the direction perpendicular to the direction in which the glaze extends, and the respective heat generating elements of the glaze are heated imagewise by energy application to heat the thermal recording layer, thereby accomplishing image reproduction.
Recently, energy which is applied to the respective heat generating elements of the glaze is controlled by a pulse-width modulation which is effected by modulating an applied time with a constant applied voltage particularly in a medical use and the like which require high image quality.
Incidentally, even if thermal heads are manufactured based on the same design values, individual thermal heads have variation in glaze heights, glaze widths, protective layer thicknesses, heater sizes and the like of actually manufactured products and it is difficult to make them to perfectly coincide with their design values. Accordingly, since the individual thermal heads have a slightly different resistance value (for example, a maximum resistance value, an average resistance value) due to the variation and the like in the heater sizes, even if the same voltage is applied to the thermal heads, they have a different current value and, as a result, a different power is applied to the heat generating elements. Further, even if the same power is applied, since a recording portion has a different temperature due to the variation in the glazes and the thermal capacities of protective layers, recording is executed in a different density. Therefore, there is a problem that even if the same voltage is applied, the density is varied by the variation in the characteristics of thermal heads such as the glaze heights, glaze widths, protective layer thicknesses, heater sizes and the like of individual thermal heads.
The variation in the densities of images among thermal recording apparatuses is a large problem when it is required to record a high quality image, in particular, when an ultra-fine middle tone image is recorded. Particularly, the variation in the densities causes an obstruction in the observation of images and is a very serious problem in the uses such as the aforesaid medical use in which high quality ultra-fine middle tone images are required because an erroneous diagnosis may be made by the variation.
When the applied power is not properly controlled by the variation in the characteristics of the thermal heads and the applied power to the heat generating elements is made excessively large, there arises a problem that thermal stress to the thermal heads is increased and the durability thereof is lowered because a peak temperature which is reached by the heat generating elements is increased accordingly. In addition, printing failure such as thermal damage and the like may be caused to a thermal recording medium (for example, the unevenness of the surface of the thermal recording medium which is caused when the surface is softened by heat). To cope with this problem, it is required to lower the thermal recording medium peak temperature in such a degree as not to hinder thermal recording to thereby prevent the imposition of an unnecessarily high voltage to the thermal head, that is, to optimize the applied voltage.
An object of the present invention is to solve the problem of prior art and provide a thermal head adjusting method of permitting a thermal head to stably record a high quality homogeneous thermal image in a thermal recording apparatus using the thermal head without being affected by difference of individual thermal heads by lowering variation in densities caused by the difference of and variation in the individual thermal heads, eliminating the damage of a thermal recording medium and improving durability by preventing the deterioration and the reduction of capability of the thermal head due to heat.
In order to achieve the above object, the invention provides a thermal head adjusting method used when an image is recorded onto a thermal recording material by applying a voltage to a thermal head in accordance with image data, comprising the steps of:
measuring the initial states of characteristic values of the thermal head; and
adjusting the voltage to be applied in accordance with the image data with respect to a reference voltage.
In a preferred embodiment, the characteristic values of the thermal head to be measured are at least one selected from a group composed of a glaze height H, a glaze width W, a protective film thickness d, a heater width l and a heater length L of the thermal head and a resistance value R of the thermal head.
In another preferred embodiment, when the reference values of the glaze height H, the glaze width W, the protective film thickness d, the heater width l, the heater length L and the resistance value R are represented by H0, W0, d0, l0, L0 and R0 and initial values measured are represented by Hi, Wi, di, li, Li and Ri, the rates of change xcex94PH, xcex94PW, xcex94Pd, xcex94Pl, xcex94PL of a power P0 which is required to make recording in a maximum necessary density at the respective initial values Hi, Wi, di, li and Li to the power P0 are determined from predetermined relationships, respectively and a voltage V determined from the following formula (1) is applied to the thermal head;                     V        =                                                                              (                                      1                    +                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              P                        H                                                                              )                                ⁢                                  (                                      1                    +                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              P                        W                                                                              )                                ⁢                                  (                                      1                    +                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              P                        d                                                                              )                                ⁢                                  (                                      1                    +                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              P                        l                                                                              )                                ⁢                                  (                                      1                    +                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              P                        L                                                                              )                                ⁢                                  R                  i                                                            R                0                                              ·                      V            0                                              (        1        )            
where, V0 is a voltage necessary to apply the power P0 to the thermal head.